A Powerglide transmission from a 1955-61 Chevy car with a 235 engine would be a lot simpler to swap, but you'd still need to do some custom fabricating of the motor mounts, driveshaft, and shift linkage. The PG is a cast iron case 2-speed, but there was a factory 235/Powerglide option, at least for cars. The TH 350 was never offered as a factory setup behind a 235 engine. Making that one work requires a custom-made flex plate, an adapter plate to mate it to the 235 engine, and a different starter. Then the crossmember at the bellhousing would need to be modified for rear motor mounts that adapt to the 350 trans. There are bellhousing mounts to adapt a TH-350 to a 55-57 Chevy passenger car, but none that I know of to do the same thing with a truck. It's going to take some custom fabrication, either by yourself of someone with the skills and equipment to do so.
Jerry
